1. Field of the Invention
The present invention relates generally to an apparatus and method for estimating Carrier-to-Interference and Noise Ratio (CMNR) in a broadband wireless communication system. More particularly, the present invention relates to an apparatus and method for estimating the CINR of an uplink channel using an uplink fast feedback signal in a Base Station (BS) in a broadband wireless communication system. CINR represents a channel quality.
2. Description of the Related Art
In a broadband wireless communication system, a BS provides a high-speed packet data service by scheduling transmission of packet data and deciding transmission parameters for the packet data based on uplink fast feedback information representing a downlink channel quality. The BS receives uplink fast feedback signals from a plurality of Mobile Stations (MSs) and selects an MS having the best downlink channel quality from among the MSs according to the uplink fast feedback signals in each time slot. The BS then determines transmission parameters for the selected MS according to its downlink channel quality and sends packet data to the MS based on the transmission parameters. The transmission parameters include data rate, code rate, and modulation order. The uplink fast feedback information include at least one of Signal-to-Noise Ratio (SNR), Carrier-to-Interference Ratio (C/I), differential SNR of each band, fast Multiple Input Multiple Output (MIMO) feedback, and mode selection feedback.
For example, an Orthogonal Frequency Division Multiple Access (OFDMA) communication system has a physical channel designated for carrying the uplink fast feedback information. Thus, an MS sends the fast feedback information to the BS on the physical channel and the BS acquires uplink channel status information from the fast feedback channel even for a non-uplink traffic transmission period of the MS.
The fast feedback channel for carrying the fast feedback information is configured as illustrated in FIG. 1 or FIG. 2, by way of example.
FIG. 1 illustrates typical 3×3 frequency-time resources allocated for reception of fast feedback information in the BS.
Referring to FIG. 1, the feedback channel is composed of six subcarrier sets 110 called tiles and, each tile includes 3×3 subcarriers on the frequency-time domain. In each of tiles 110, eight surrounding subcarriers carry modulation symbols and one center subcarrier carries a pilot symbol.
FIG. 2 illustrates typical 4×3 frequency-time resources allocated for reception of fast feedback information in the BS.
Referring to FIG. 2, the feedback channel is composed of six subcarrier sets 210 called tiles, and each tile includes 4×3 subcarriers on the frequency-time domain. In each of tiles 210, four corner subcarriers carry pilot symbols and the other eight subcarriers carry modulation symbols.
The BS can control the power of the uplink channel by estimating its CINR using the uplink fast feedback information. Without successful uplink power control, interference becomes severe between cells. Due to the resulting degradation of link performance or unstable communication status, Quality of Service (QoS) cannot be satisfied. As a consequence, the decrease of data rate leads to the decrease of cell throughput. Accordingly, there exists a need for a method that can reliably estimate CINR in the broadband wireless communication system.
To estimate CINR of the uplink channel using the uplink fast feedback information received from the MS, the BS first calculates the soft-decision values of symbols of the uplink fast feedback information. The BS correlates the soft-decision values with each codeword, squares the absolute values of the correlations, and sums the squares. Then the BS selects a codeword with the largest sum (hereinafter, referred to as a maximum codeword) from among given codewords and detects information data bits corresponding to the codeword. Each codeword is composed of orthogonal vectors having values as illustrated in FIG. 3.
FIG. 3 illustrates typical orthogonal vectors used for modulation. Referring to FIG. 3, when the BS uses Quadrature Phase Shift Keying (QPSK), orthogonal vectors are formed using QPSK symbols,
      P    ⁢                  ⁢    0    ⁢          (              exp        ⁡                  (                      j            ⁢                          π              4                                )                    )        ,
      P    ⁢                  ⁢    1    ⁢          (              exp        ⁡                  (                      j            ⁢                                                  ⁢                                          3                ⁢                π                            4                                )                    )        ,      P    ⁢                  ⁢    2    ⁢          (              exp        ⁡                  (                                    -              j                        ⁢                                          3                ⁢                π                            4                                )                    )        ,      and    ⁢                  ⁢    P    ⁢                  ⁢    3    ⁢                  (                  exp          ⁡                      (                                          -                j                            ⁢                              π                4                                      )                          )            .      
After detecting the maximum codeword, the BS calculates the received power level or strength and noise power level of the received signal using the squared absolute correlation values of the received signal with respect to the maximum codeword and then calculates the CINR of the received signal based on the received power and noise power levels. For example, the BS calculates the received power level by averaging the squared absolute correlation values of subcarriers included in the tiles of the received signal with respect to the maximum codeword.
Also, the BS calculates the noise power strength by calculating the difference between every adjacent two modulation symbols correlated with the maximum codeword, squaring the absolute values of the differences for all six tiles, and averaging the squares. While the noise power strength is estimated based on the differences between adjacent correlated modulation symbols, it is to be clearly understood that correlated pilot symbols can be used instead of the correlated modulation symbols in estimation of the noise power strength.
Using the received power and noise power levels, the BS calculates the CINR according to Equation (1),
                    CINR        =                                                                              1                                      number                    ⁢                                                                                  ⁢                    of                    ⁢                                                                                  ⁢                    tiles                    ×                    number                    ⁢                                                                                  ⁢                    of                    ⁢                                                                                  ⁢                    FF                    ⁢                                                                                  ⁢                    symbols                                                                                                                                                                  ∑                                              m                        =                        1                                                                    number                        ⁢                                                                                                  ⁢                        of                        ⁢                                                                                                  ⁢                                                  tiles                          ⁡                                                      (                                                          =                              6                                                        )                                                                                                                ⁢                                                                  ∑                                                  k                          =                          1                                                                          number                          ⁢                                                                                                          ⁢                          of                          ⁢                                                                                                          ⁢                          FF                          ⁢                                                                                                          ⁢                                                      symbols                            ⁡                                                          (                                                              =                                8                                                            )                                                                                                                          ⁢                                                                                                                              Z                                                          m                              ,                              k                                                                                                                                2                                                                              -                                      P                    N                                                                                            P            N                                              (        1        )            where number of FF symbols represents the number of fast feedback modulation symbols per tile, Zm,k represents the correlated received signal, and PN represents the noise power strength.
The correlated received signal is expressed as Equation (2),Zm,k=Cm,k×Ym,k=Hm,k+Cm,k×Nm,k, 1≧m≧number of tiles, 1≧k≧number of FF symbols  . . . (2)where number of FF symbols represents the number of fast feedback modulation symbols per tile, Cm,k represents a code symbol in a fast feedback orthogonal vector, Hm,k represents a channel coefficient, Nm,k represents a noise component, and Ym,k represents a received signal on a kth subcarrier in an mth tile, expressed as Ym,k=Cm,kHm,k+Nm,k.
The noise power strength is computed by Equation (3),
                              P          N                =                                                            1                                                                                                                                                                                        2                              ×                                                                                                                                                                                          number                              ⁢                                                                                                                          ⁢                              of                              ⁢                                                                                                                          ⁢                              tiles                              ×                                                                                                                                                                                                                              (                                                                              number                            ⁢                                                                                                                  ⁢                            of                            ⁢                                                                                                                  ⁢                            FF                            ⁢                                                                                                                  ⁢                            symbols                                                    -                          1                                                )                                                                                                                                                                                      ∑                                      m                    =                    1                                                        number                    ⁢                                                                                  ⁢                    of                    ⁢                                                                                  ⁢                    tiles                    ⁢                                                                                  ⁢                                          (                                              =                        6                                            )                                                                      ⁢                                                      ∑                                          k                      =                      1                                                                                      (                                                                              number                            ⁢                                                                                                                  ⁢                            of                            ⁢                                                                                                                  ⁢                            FF                            ⁢                                                                                                                  ⁢                            symbols                                                    -                          1                                                )                                            ⁢                                              (                                                  =                          7                                                )                                                                              ⁢                                                                                                                                    Z                                                      m                            ,                            k                                                                          -                                                  Z                                                      m                            ,                                                          k                              +                              1                                                                                                                                                                  2                                                                                                          (        3        )            where number of FF symbols represents the number of fast feedback modulation symbols per tile, Zm,k represents the correlated received signal, and |Zm,k−Zm,k+1|2 represents the squared absolute difference between adjacent correlated modulation symbols.
As described above, the BS estimates the CINR using the uplink fast feedback information. Yet, it uses only modulation symbols or pilot symbols without fully utilizing all information of the subchannel of the MS, thereby decreasing the reliability of the CINR.
Accordingly, there is a need for a method that can increase the detection efficiency of fast feedback information and reliably estimate CINR in the BS.